114 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
stripped the lower plateaus on the south and east (a, p. 22). It was 
after the great Miocene denudation that the Kaibab and the Echo cliffs 
monoclines began to have a separate existence (a, p. 42, c, pp. 192, 205), 
and with these local uplifts there was also a wide-spread and sudden up- 
lifting of the whole district in virtue of which the deep and narrow 
canyons of to-day have been cut (a, pp. 38, 45, c, pp. 100, 191); it is by 
this uplift of late date that the high plateaus of Utah have reached 
their great altitude (a, p. 23). The system of north and south faults, 
extending from the Grand canyon northward into the high plateaus, was 
closely associated with the Pliocene uplift of the region, although some 
of them may have begun earlier and continued later, even as late as the 
glacial period (a, pp. 27, 28, 35, ¢, pp. 94, 116, 187, 191, 226). It was 
during a pause in the Pliocene uplift that the esplanade of the ‘‘ outer 
gorge” in the Kanab and Uinkaret plateaus was opened (c, pp. 121, 
226, 227), while the inner gorge of the Grand canyon in these plateaus 
has been eroded since the later faulting took place (a, p. 37, c, pp. 94, 
227). Although some superposed streams are found in the Water- 
pocket flexure, where the Tertiary strata have been stripped from it 
(a, p. 288), the water-ways are thought to be antecedent in nearly 
all cases (a, pp. 16, 17, ¢, pp. 73, 187, 204, 219); San Rafael river and 
Curtis creek, both of which cross the San Rafael swell, are instanced as 
particularly good examples of their class (4, p. 63). Most of the great 
Miocene denudation was performed in a humid climate; then there was 
a gradual desiccation, as a result of which a number of tributary 
streams disappeared in Pliocene time, leaving untrenched valley floors 
high above the present canyon bottom (ce, pp. 99, 194, 201) ; to the sur- 
viving streams of this dry period belongs the erosion of the narrow can- 
yons and the steep cliffs (a, pp. 22, 23, c, pp. 223, 227). A temporary 
return to a humid climate occurred in the glacial period, and certain 
ravines on the Kaibab and Paria plateaus were then carved (c, pp. 196, 
202). 
In brief, the work of denudation in the Grand canyon district seems 
to require two cycles of erosion: the first, initiated by a broad uplift, 
had advanced well towards old age under a humid climate when 
the second, characterized by an arid climate, was introduced by another 
broad uplift complicated by certain dislocations ; the second was sub- 
divided into two episodes by a pause in the uplift, the first episode having 
approached maturity, while the second has not passed beyond youth. 
The work of other observers, chiefly Gilbert, Howell, and Walcott, 
will be referred to further on. 
